The present invention relates to electrochromic devices incorporating a security system. More particularly, the present invention relates to electrochromic devices particularly useful as transparencies which include a security feature to detect breakage of the transparency.
Electrochromic devices are well-known in the art for use in various applications. Such electrochromic devices typically include a sealed chamber defined by two pieces of glass which are separated by a gap or space which contains an electrochromic medium. The electrochromic medium typically includes anodic compounds and cathodic compounds together in a solution. The glass substrates typically can include transparent conductive layers coated on facing surfaces of the glass and in contact with the electrochromic medium. The conductive layers on both glass substrates are connected to electronic circuitry. When the conductive layers are electrically energized, an applied potential is introduced into the chamber of the device which electrically energizes the electrochromic medium and causes the medium to change color. For example, when the electrochromic medium is energized, it may darken and begin to absorb light.
Electrochromic devices are used in rear-view mirror assemblies in automotive applications. In such uses, a photocell can be incorporated into the electrochromic cell to detect a change in light reflected by the mirror. When a specific level of light is reflected, for instance when lights are reflected at night, the photocell can activate to apply an electrical potential to the electrodes in the cell, thus causing the electrochemical medium to change color and create a darkening effect, thereby dimming the mirror to the lights. Additionally electrochromic devices have been proposed for use in other automotive applications, such as windshields and windows, as well as other window transparency applications, such as aircraft window assemblies and architectural windows.
Security alarm systems for conventional windows are known, and such conventional alarms typically include a frangible conductor, such as a foil tape or thin wire, typically in a serpentine or circumscribing path, affixed to the window glazing and through which an electric current is passed. When the window is broken, the conductor also breaks thereby opening the circuit and triggering an alarm. A typical system employing thin wires may be seen in U.S. Pat. No. 1,223,583, and a typical system employing foil tape may be seen in U.S. Pat. No. 3,409,886. Moreover, transparent conductive coatings can be utilized in such window glazings for passing current through the entire window area, such as set forth in U.S. Pat. No. 3,441,925. U.S. Pat. No. 3,947,837 provides a window glazing including a grid of fine wires which permits resetting of the grid after a partial break occurs. U.S. Pat. No. Re. 33,343 provides a crack detector for an electrically conductive windshield which monitors the windshield to determine a change in the conductance of the windshield due to a break in the windshield and interrupts power to the conductive panel in the event of such a break.
An object of the present invention is providing electrochromic devices which are useful in window assemblies, which can be easily manufactured and which provide security features.
The present invention is directed to an electrochromic security window assembly capable of reduced light transmittance upon application of electrical potential thereto and which includes a security alarm system which is capable of detecting a break in the window assembly. The window assembly includes first and second spaced transparent substrates defining a chamber therebetween. Both of the first and second substrates include first and second conductive coatings on respective facing surfaces thereof, and means for applying electrical potential to the first and second conductive coatings. An electrochromic medium having reduced light transmittance upon application of electrical activation like an electrical potential thereto is contained within this chamber. The electrochromic medium, which can be a solution, gel, polymer or polymers, preferably includes at least one anodic electrochromic compound and at least one cathodic electrochromic compound. At least one of the anodic and cathodic electrochromic compound is a dye. Preferably, the cathodic electrochromic compound is a viologen compound and the anodic electrochromic compound is a phenazine compound.
The assembly further includes circuit means connected to at least one of the first and second conductive coatings, which is capable of detecting a change in an electrical characteristic of the at least one conductive coating connected thereto, and for generating an output signal in response to a change in the electrical characteristic. Such a change in electrical characteristic, such as a change in the resistance or current flow through the circuit, corresponds to a break in the at least one conductive coating.
Preferably, the means for detecting the change in the electrical characteristic detects a change in current through the at least one conductive coating. The electrical potential applied to the at least one conductive coating is adjustable. The potential can be between a voltage below the reduction potential capable of simultaneously oxidizing the anodic electrochromic compound and reducing the cathodic electrochromic compound when the window assembly is in a state of increased light transmittance and a voltage above the reduction potential capable of simultaneously oxidizing the anodic electrochromic compound and reducing the cathodic electrochromic compound when the window assembly is in a state of reduced light transmittance or is in a transition from a state of increased light transmittance to a state of reduced light transmittance. The security alarm feature can be activated when the electrochromic window assembly is in a clear state or a darkened state, or in a transition period between the clear state and the fully darkened state. Alternatively, the electrical potential can be intermittently applied to the at least one conductive coating connected to the circuit means at a voltage above and below the reduction potential capable of simultaneously oxidizing the anodic electrochromic compound and reducing the cathodic electrochromic compound, thereby providing a pulsing type current flow.
In a further embodiment, the electrochromic window assembly includes an electrical potential applicator or means for alternately applying the electrical potential: (i) between the first and second conductive coatings at a voltage above the reduction potential capable of simultaneously oxidizing the anodic electrochromic compound and reducing the cathodic electrochromic compound when the window assembly is in a state of reduced light transmittance; and (ii) the at least one conductive coating connected to the circuit means at a voltage capable of detecting a change in the electrical characteristic. Such an arrangement also involves an intermittent pulsing of the current when the window assembly is in the darkened state.
In a further embodiment, a method for monitoring penetration or breakage of a window assembly which is capable of variable light transmittance is provided. In the method, an electrical characteristic of the at least one conductive coating having the electrical potential applied thereto as described above is monitored for a change in the electrical characteristic. Upon detecting a change, an output signal is generated and is sent, for example, to an alarm and/or a global positioning system (GPS).
Preferably, the monitoring of the change in the electrical characteristic includes monitoring a change in current through at least one conductive coating.